Method and apparatus for actuating an electric circuit

ABSTRACT

A field of electrostatic, electromagnetic or high frequency radiant energy is provided on a predetermined intermittent cycle in a confined space through which persons are directed. A tuned resonant circuit, concealed on merchandise being carried through the space, is activated by the energy field. During the time interval when the energy field is cut off, the decaying electric signal from the tuned resonant circuit is radiated to a receiver. The received electric signal functions to activate an alarm.

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States Patent 1191 Thompson et al.

1111 3,740,742 1 1 June 19,- 1973 METHOD AND APPARATUS FOR 1 ACTUATINGAN ELECTRIC CIRCUIT [76] Inventors: Thomas F. Thompson, 3660 HarlowRoad, Eugene, Oreg. 9740]; Joseph W. Griffith, 8705 S. W. White Court,Portland, Oreg. 97225 22 Filed: May 11, 1971 21 Appl. No.: 142,132

Related U.S. Application Data [63] Continuation-impart of Ser. No.879,080, Nov. 24, 1969, abandoned, which is a continuation-in-part ofSer. No. 797,053, Feb. 6, 1969, abandoned.

[52] U.S. Cl 340/280, 340/258 C, 340/408,

' I 340/224, 343/6.8 R [51] Int. Cl. G08b 13/24 [58] Field of Search340/280, 258 D, 258 B,

340/258 C, 408, 224, 152 T; 325/29, 8; 128/21 A; 343/68 R [56]References Cited 7/1959 Gille 343/68 R 3,218,638 11/1965 Honig 340/2242,774,060 12/1956 Thompson..... 340/280 2,818,732 1/1958 Bennett 340/2243,582,931 6/1971 Nawrocki... 340/258 3,493,955 2/1970 Mi'nasy PrimaryExaminer-D0na1d J. Yusko Assistant ExaminerGlen R. Swarm, IIIAttorney-Oliver D. Olson [5 7] ABSTRACT A field of electrostatic,electromagnetic or high frequency radiant energy is provided on apredetermined intermittent cycle-in a confined space through whichpersons are directed. A tuned resonant circuit, concealed on merchandisebeing carried through the space, is activated by the energy field.During the time interval when the energy field is cut off, the decayingelectric signal from the tuned resonant circuit is radiated to areceiver. The received electric signal functions to activate an alarm.

20 Claims, 15 Drawing Figures CLIPPER CLlPPER ,L ALARM PAIENTEB JUN!91975 SHEET 3 BF 3 AMPL. BIAS TIMING SIGNALS I E238 '272 l I E J in FIG.I4 270 SCHMIDT I TRIGGER f I 250 i 264 266 BIAS 252 TIMING SIGNALSSCHMIDT I TRIGGER ALARM COINCIDENCE AMPL- GATE I I 2 NED THOMAS F.THOMPSON REFE E E JOSEPH w GRIFFITH FIG.

INV NTQRS. BY

AGENT METHOD AND APPARATUS FOR ACTUATING AN ELECTRIC CIRCUIT CROSSREFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION Thisinvention relates to the actuation of electric ciredits, and moreparticularly to a novel method and apparatus for actuating an" electriccircuit from a remote position. Specially, this invention relates to amethod and apparatus for detecting stolen merchandise.

Various methods and devices have been employed heretofore to effectactuation of an electric circuit from a remote position. Among these isthe method and apparatus disclosed in applicants earlier U.S. Pat. No..

2,774,060 over which the present invention represents an improvement.The method and apparatus of applicants earlier patent has manyfunctions, including the detection of stolen merchandise, and involvesthe use of a tuned resonant circuit which, when placed in the field ofan oscillator causes the oscillator to produce a change inpotentialzwhich may be utilized to actuate an electric circuit. v i

Although apparatus of the type disclosed in applicants earlier patent isquite effective for the purposes intended, its effectiveness is somewhatlimited by the shielding effects of extraneous objects placed inproximity to, the tunedcircuit, thereby limiting the effective range ofoperation of the apparatus.

Other methods and devices of the class described are generallycharacterized by being operable with a multiplicity of magnetic orelectrically conductive objects of various shapes and sizes. Althoughsuch methods and apparatus may find utility in the actuation of certaintypes of electric circuits,they are of no value forvthe purpose ofdetecting stolen merchandise.

SUMMARY OF THE INVENTION In its basic concept the present inventioninvolves the activation ofa tuned-electric energy absorbing andradiating device-by an intermittently generated field of electrostatic,electromagnetic or radio frequency radiant energy, whereby when theenergy field is cut off the decaying electric signal from thetuneddevice is radiated to a receiver. The receiver electric signal functionsto activate an electric circuit.

It is by virtue of the foregoing basic concept that the principalobjective of the present invention is achieved, namely to overcome thedisadvantages of prior methods and apparatus as described hereinbefore.

Another important object of the present invention is to provideapparatus of the class described in which the tuned device is a passivetuned resonant LC circuit in which means is provided for disabling thetuned resonant circuit after it has served its purpose.

A furtherimportant object of this invention is the provision of a noveltuned resonant circuit construction for use with the method andapparatus.

The foregoing and other objects and advantages of the present inventionwill appear-from the following detailed description, taken in connectionwith the accompanying drawings of-the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic electricaldiagram, partly in block form, illustrating the method and one form ofapparatus of the present invention.

FIG. 2 is a graphic representation of a plurality of waveforms ofelectric signals illustrating the operation of the apparatus of FIG. 1.

FIG. 3 is a plan view'of a merchandise tag having concealed therein, andillustrated by broken lines, a tuned resonant circuit embodying featuresof the present invention.

FIG. 4 is a fragmentary sectional view on a magnified scale taken on theline 44 in FIG. 3.

FIG. 5 is a schematic representation of means for disabling the tunedresonant circuit illustrated in FIGS. 3 and 4.

FIG. 6 is a schematic electrical diagram, partly in block form,illustrating the method and a second form of apparatus of the presentinvention.

FIG. 7 is a graphic representation of a plurality of waveforms ofelectric signals illustrating the operation of the apparatus of FIG. 6.7

FIG. 8 is a schematic electrical diagram, partly in block form,illustrating the method and a third form of apparatus of the presentinvention.

FIG. 9 is a schematic electrical diagram, partly in block form,illustrating a still further modified form of apparatus embodying thefeatures of this invention.

FIG. 10 is aplan view of an identification card having incorporatedtherewith means by which to activate the apparatus shown in FIG. 9.

FIG. 11 is a schematic diagram of another form of tuned resonant circuitconcealed in a tag illustrated by brokenlines and embodying features ofthis invention.

FIGS. 12 and 13 are schematic representations of further modified formsof tuned resonant circuits embodying features of this invention.

FIG. 14 is a schematic electrical diagram, partly in block form,illustrating a still further modified form of apparatus embodying thefeatures of this invention.

FIG. 15 is a schematic electrical diagram in block form illustrating amodification of the receiving component of the apparatus of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 ofthe drawings there is illustrated a pair of electrically conductiveplates 10 and 12. These plates are separated a distance sufficient todefine a space between them operable to receive at least one energyabsorbing and radiating device, such as tuned resonant circuit 14. Thus,the plates may be spaced apart on opposite sides of a conveyor on whichmerchandise may travel. The spacing between plates alternatively maydefine an aisle in a department store or the like through which thecustomers must pass. As a still further alternative, the plates may bepositioned adjacent the opposite walls of a room, whereby to activateall of the tuned resonant circuits carried by articles of merchandisecontained in the room.

One of the plates 10 is connected to the positive terminal of a source16 of direct current. The negative terminal of the source is connectedto the plate of the switching tube 18, the cathode of which is grounded.The control grid of the tube is connected through the capacitor 20 tothe output of the clipper circuit 22,

many types of which are well known in the art. The input of the clippercircuit is connected across the secondary winding 24 of the transformer26. One end of this secondary winding is connected to ground. Theprimary winding 28 is connected to a source of alternating current,conveniently the 60 cycle, 110 volt conventional household source.

The other electrostatic plate 12 is connected to the negative terminalof the source 30 of direct current. The positive terminal of the supplysource is connected to the cathode of the switching tube 32, the plateof which is connected to ground. The control grid of the tube isconnected through the capacitor 34 to the output of the clipper circuit36 the input of which is connected across the ungrounded secondarywinding 38 of the transformer 26.

The positive electrostatic plate also is connected to the plate of theswitching tube 40. The negative plate 12 is connected through theadjustable potentiometer 42 to the cathode of the switching tube 40.This cathode also is connected to one terminal of the source 44 ofdirect current. The other terminal of the supply source and the controlgrid of the switching tube 40 are connected through the capacitor 46 tothe output of the clipper circuit 48. Although one clipper circuit maybe sufficient, it is preferred that two or more clipper circuits beemployed to provide the desired signal waveform described hereinafter.Thus, the input of the clipper circuit 48 is connected to the output ofthe associated clipper circuit 50 the input of which is connected acrossthe secondary winding 52 of the transformer 26. The grounded end of thiswinding is the same as that of winding 24.

The secondary winding 54 of the transformer 26 is connected across theinput of the clipper circuit 56 the output of which is connected to theradio frequency receiver 58 to effect activation of the latter. Forexample, the clipper output may be connected to provide bias voltage forthe first r.f. stage of the receiver. The receiver antenna 60 functionsto receive a corresponding radio frequency signal from a tuned resonantcircuit 14, as described hereinafter, whereby to activate an alarm L orother electric load connected to an appropriate output of the receiver.For example, the load may be in an electric circuit controlled by arelay, the coil of which is connected through a rectifier to the outputof the if stage of the receiver.

Although the resonant circuit 14 may be provided in a variety of formswell known in the art, the construction illustrated in FIGS. 3 and 4 ispreferred. In this construction the resonant circuit is concealedbetween laminated sheets 62 and 64 of paper, or other electricallynon-conductive material, as follows: On the inside face of one sheet 62there is formed an open loop 66 of electrically conductive metal such ascopper, silver, etc. This may be provided by the well known technique ofprinted circuitry. On the inner side of the other sheet 64 there issimilarly formed an arcuate segment 68 of the electrically conductivematerial. This segment is positioned on the sheet so that when thelatter is superimposed over the first mentioned sheet 62, the arcuatesegment 68 overlaps one end portion of the open loop 66. lnterposedbetween these overlapped portions is a film 70 of electrical insulationmaterial. This may be provided in the form of a small plate bonded toone of the overlapping portions, or it may be in the form of a liquidmaterial, such as thermoplastic resin, painted or otherwise depositedover the surface of one of the overlapping portions.

The arcuate segment 68 of electrically conductive material may be ofsufficient length to overlap the opposite end of the open loop 66 toprovide electrical conductivity therebetween when the two sheets arebonded together in superimposed relation. However, in the embodimentillustrated, the arcuate segment terminates short of the opposite end ofthe loop and electrical connection is provided by a length of lead orother electrically conductive fusible material 72. This length offusible material preferably is of small cross section, such as a thread,and it may be formed as an extension of the arcuate segment, asillustrated, or as an extension of the opposite end of the open loop. Ineither case it functions as a fusible electrical connection between thearcuate segment and the end of the open loop opposite the overlappedportions.

The fusible connection serves the additional function of enabling thedestruction of the tuned resonant circuit after it has served itspurpose. Thus, referring to FIG. 5, when it is desired to destroy thetuned circuit it is placed in the field of the output coil 74 of anoscillator 76 which provides sufficient current to melt the fusibleconnection.

The L-C tuned circuit 14 is designed to provide a Q factor of sufficientmagnitude as to effect appropriate activation of the receiver outputcircuit. Although the Q factor may vary over a considerable range,depending in part upon the sensitivity of the receiver, a Q factor ofabout 80 is satisfactory. Improvement of the Q factor of the tunedcircuit may be achieved by depositing particles 78 (FIG. 3) of polyironor other suitable material within the loop 66 prior to bonding the twosheets 62 and 64 together.

Having formed the segments 66 and 68 of the tuned circuit on the innersurfaces of the sheets, the latter are bonded together in superimposedrelation, by means of a suitable adhesive, with the overlapped portionsproperly oriented and with the insulating layer between them. Thecompleted tag is thin and flexible and may be used in the manner of aconventional price tag upon which appropriate pricing information may beprinted.

The dimensions of the tuned circuit, and hence of the confining tag, maybe varied over a considerable range, as desired. For example, the tunedcircuit may be designed to have a resonant frequency of 10 megacycles,whereby the diameter of the loop will be about one-half inch. This mayprovide a tag of about three-quarters inch square.

The resonant frequency of the tuned circuit also may be selected from awide range of radio frequencies most suitable for the use intended. Thelower the frequency the larger the dimensions of the tuned circuitbecomes but the lesser are the shielding effects imposed by extraneousobjects. Thus, very low or very high radio frequencies may be employedfor certain uses. For use with merchandise price tags a frequency ofabout 10 megacycles is quite satisfactory, although other frequenciesmay be employed.

In the operation of the apparatus illustrated in FIG. 1 let it beassumed that the electrostatic plates 10 and 12 are positioned onopposite sides of an aisle, downstream of a cashiers counter to whichcustomers are directed. The receiver antenna 60 also is positioneddownstream from the cashiers counter. Let it also be assumed that eachpiece of merchandise in the store has attached to it one of the tagsconcealing a tuned resonant circuit l4. The tag may be concealed withinthe merchandise, or it may take the form of a price tag attached in avisible place to the merchandise.

As a customer passes through the aisle and completes his purchases withthe cashier, the cashiereither removes the tag from each piece ofmerchandise, or subjects the tuned circuit 14 to destruction by placingit in the field of the oscillator coil 74. Conveniently, this coil maybe placed under the surface of the cashiers counter so that the tunedcircuit will be destroyed merely by placing the merchandise on thecounter. In either case the customer then may exit along the aislebetween the electrostatic plates and 12 and past the receiver antenna 60without incident.

Assume now that a person has concealed an article of merchandise withthe intent of stealing it. Having completed his purchase of otherarticles at the cashier counter, he proceeds to exit from the aisle bypassing between the plates 10 and 12 and by the receiver antenna 60. Theapparatus then operates as follows:

The electrostatic plates 10 and 12 are charged positively andnegatively, respectively, as indicated in FIG. 1, on an intermittenttime cycle determined by the frequency of the alternating current supplyto the primary winding 28 of the transformer. The waveform 80 (FIG.

2) illustrates the in-phase outputs of the clipper circuits 22 and 36,and when the voltage reaches the level indicated by' the dash line82'the switching tubes 18 and 32 are caused to conduct. Such conductionfunctions to complete the electric circuits of the direct current supply sources 16 and 30, whereby to apply the corresponding charges'to theplates 10 and 12. The time interval T of conduction of the switchingtubes is governed by the frequency of the alternating current supply atthe primary winding, and this conveniently may be 60 cycles per secondas mentioned hereinbefore.

The waveform 84- represents the output of the final clipper unit 48.This waveform is in phase opposition to waveform 80 by virtue of theopposed ground connections of windings 24 and 52. When the potentialreaches the level indicated by the dash line 86 the switching tube 40 iscaused to conduct. Conduction of this tube functions to provide anelectrical short between the plates 10 and 12, thereby acceleratingtheir discharge and collapse of the electrostatic field. Thepotentiometer 42 is adjusted to provide complete collapse of theelectrostatic field in the time interval T between cutoff of conductionof the switching tubes 18,

32 and the attainment of maximumconduction of the switching tube 40.This time interval is chosen to represent one-half cycle of the resonantfrequency of the tuned circuit 14.

The waveform 88 represents the output of the clipper 56. This waveformis in phase with waveform 84. When the potential reaches the levelindicated by the dash line 90 the receiver 58 is activated. The timeinterval T between conduction of the switching tube 40 .and activationof the receiver provides sufficient time delay to insure againstreception by the receiver of false signals due to collapsing of theelectrostatic field and to insure reception by the receiver only of thesignal radiated by the tuned circuit 14.

The damped waveform 92 represents the decaying radio frequency signalradiated by the tuned circuit. The decay of this signal commences uponcomplete collapse of the electrostatic field and, depending upon the Qfactor of the tuned circuit, continues for a time T following activationof the receiver. The decaying signal following activation of thereceiver activates the output circuit of the receiver to provide anelectric signal to activate the alarm L. The alarm may be in the form ofa lamp, a buzzer, a camera actuator and/or any other device suitable forthe purpose intended.

Referring now to the embodiment illustrated in FIG. 6: The pair ofelectrostatic plates 10 and 12 previously described are replaced byelectrically conductive coils and 102 to provide an electromagneticfield between them. In the preferred embodiment illustrated,

each of the coils of the pair consists of two'or more coils connectedtogether in parallel to provide a very low Q factor.

In manner similar to the electrostatic plates, the pair of coils arespaced apart to define between them a space in which one or more tunedresonant circuits 14 may be placed for excitation on an intermittentcycle.

The coils are connected at one end to ground and at the other end to oneterminal of a source 104 of direct current. The other terminal of thesupply source is connected to the plate of the switching tube 106. Thecontrol grid of the tube is connected through the capacitor 108 to theoutput of the clipper circuit 110.'The input of the clipper unit isconnected across the secondary winding 112 of the transformer 114 theprimary winding 116 of whichis connected to a suitable source ofalternating current as, for example, conventional 60 cycle householdcurrent. The other secondary winding 118 of the transformer is connectedacross the input of the clipper circuit 120 the output of which isconnected to the receiver 122 for activating the latter on anintermittent cycle, 180 out of the phase with activation of theswitching tube 106. As in the embodiment previously described, this isachieved by grounding the secondary windings 112 and 118 at oppositeends, as illustrated. The antenna 124 of the receiver functions toreceive the signal radiated by the tuned resonant circuit 14 to effectactivation of an alarm L.

Referring now to P16. 7, the waveform represents the output of theclipper circuit 110. When the potential reaches the level indicated bythe dash line 132, the switching tube 106 is caused to conduct, therebycompleting the electric circuit of the direct current supply 104 andenergizing the coils 100 and 102 for the time interval T of conduction.The waveform 134 represents the output of the clipper circuit 120. Whenthe potential reaches the level indicated by the dash line 136, thereceiver 122 is activated for the time interval T The interval T of timebetween cutoff of conduction of the switching tube 106 and activation ofthe receiver 122 provides suff cient time delay to insure completecollapse of the electromagnetic field and thus insure reception by thereceiver only of the decaying signal 92 radiated by the tuned circuit14, as explained hereinbefore.

In the embodiment illustrated in HO. 8 the electrostatic plates of FIG.1 and the electromagnetic coils of FIG. 6 are replaced by transmitterand receiver antennas and 142, respectively, spaced apart sufficientlyto provide the desired energy field for activating one or more tunedresonant circuits 14. The transmitting antenna is associated with atransmitter 144, the input of which is connected to the output of theclipper circuit 146. The input of the clipper circuit is connectedacross the secondary winding 148 of the transformer 150, the primarywinding 152 of which is connected to a suitable source of electricpotential, as explained hereinbefore. The receiving antenna 142 isassociated with a receiver 154 the input of which is connected to theoutput of the clipper circuit 156 having its input connected across thesecondary winding 158 of the transformer 150. As in the previousembodiments, these transformer secondary windings are grounded atopposite ends so that the output waveforms from the clipper circuitsactivate the transmitter and receiver during different portions of thecycle, substantially in the manner illustrated by the waveforms of FIG.7. Accordingly, a portion of the decaying signal 92 radiated by thetuned circuit 14 after the transmitter has been cut off is received bythe receiver to activate an alarm L. It will be understood that thetransmitter and the receiver are designed to operate on the samefrequency as the resonant frequency of the tuned circuit 14.

FIG. 9 illustrates a further modified form of apparatus wherein threereceivers 160, 162 and 164 each are tuned to a different frequency forassociation with a correspondingly tuned resonant circuit 166, 168 and170, respectively, as is illustrated on the card 172 in FIG. 10. Theoutputs of the receivers are connected one to each of the relay coils174, 176 and 178, respectively, the switch contacts 180, 182 and 184 ofwhich are arranged in series in the electric circuit of a device L to beactuated. Accordingly, it will be apparent that in order to actuate thedevice all three tuned circuits must be present at one instant withinthe field of the antennas of the three receivers.

The apparatus of FIG. 9 may be employed for the detection of stolenmerchandise, as well as for many other purposes. For example, it may beadapted for use in actuating a door lock, such as in private clubs,defense plants, parking lots and various other places where privacy orsecrecy is to be maintained. For this purpose each person authorized toenter a restricted area is provided with an identification card ormembership card, such as the card 172 illustrated in FIG. 10. As amatter of additional security, such cards may be changed from time totime to add or subtract tuned circuits, or to change the frequencycharacteristics thereof, in order to render previously issued cardsobsolete. It will be understood, of course, that the receivers also willbe adjusted to correspond to the frequencies of the tuned circuits inthe newly issued cards.

In the embodiments previously described both the source of electricenergy field and the receiver necessarily must have the characteristicthat once they are turned off they must have no residual stored energyduring the time of radiation of the decaying signal from the tunedcircuit 14 or other energy absorbing and radiating device. Further, thesource of intermittent electrical energy field must have thecharacteristic of cutting off in a time substantially less than the timeduring which the decaying signal is being radiated. The embodiments ofFIGS. 14 and illustrate specific apparatus providing thesecharacteristics.

Referring first to FIG. 14, the transistor 220 and transformer 222represent an oscillator. One winding 224 of the transformer is connectedto the transmitting antenna 226, a second winding 228 connects a sourceof direct current potential to the collector of the transistor, and thethird winding 230 connects the base of the transistor to a source 232 ofelectric timing signals through an amplifier 234 and bias circuit 236.The timing signals may be provided by means of a conventional pulsegenerator, or other well known form of device providing timing signals,in the manner of the transformer and clipper arrangement describedhereinbefore.

The desired frequency of oscillation of the transmit ter is establishedby providing a tuned reference elecv trode. In the embodimentillustrated this is provided by a low impedance resonant device 238 inthe form of the series arrangement of inductor 240 and capacitor 242connecting thetransistor emitter to ground. The inductor and/orcapacitor are adjustable in order to establish the desired frequency oftransmission providing the intermittent electric energy field.

Means is provided for insuring that the transmitter will be cutoff in atime substantially less than the time during which the decaying signalof the energy absorbing and radiating device 14 is being radiated. Forthis purpose, the capacitor 242 or, preferably, the inductor 240 isshunted by a resistor 244 of an appropriate value capable of reducingthe effective Q of the series tuned circuit, i.e., reducing the energystoring capability of the inductor to a very low value. In this regard,at all frequencies other than that determined by the LC circuit, theimpedance of the emitter circuit is high and therefore it has a highdegenerative capability for all such other frequencies. However, at thefrequency established by the LC circuit, the series impedance is low andthe emitter is essentially at ground potential. Accordingly, any signalappearing on the base of transistor, by virtue of the transformer,becomes amplified.

In a typical application, the direct current resistance of the inductor240 is, for example about 0.1 ohm, but at a frequency of for examplemegacycles it is about 10,000 ohms, and the series tuned impedance ofthe LC circuit at the resonant frequency is about 0.1 ohm. A shuntingresistor 244 of about 200 ohms provides satisfactory results.

Means other than the shunting resistor 244, such as a ferrite bead, maybe associated with the LC circuit for absorbing energy from the circuitand thereby reducing its effective Q.

The source of intermittent electric energy field may be obtained bymeans other than the transmitter previously described. For example, itmay be a pulse transmitter providing a single spike of energy, or asaturable reactor, or any other type of device providing theamplification and phase shift requirements for an oscillator.

The LC circuit previously described is but one of many other suitableforms of series tuned devices capable of use for purposes of thisinvention. Crystals, mechanical filters and other low impedance resonantdevices are examples. In any event, the source of intermittent electricenergy field must be characterized by the absence of residual storedenergy when turned off, as previously explained.

The receiver component of the apparatus illustrated in FIG. 14 is aselective energy detector synchronized with the operation of thetransmitter by means of timing signals from the timer 232. These signalsare fed through a bias circuit 246 and the winding 248 of an untunedtransformer 250 ofa receiving antenna 252, to the base of amplifiertransistor 254. The emitter of the transistor is connected to groundthrough a low impedance resonant device 256, again in the form of theseries arrangement of inductor 258 and capacitor 260, in manner similarto the series tuned LC circuit previously described, with the inductorbeing shunted by the resistor 262. Thus, the low impedance series tunedLC circuit functions as part of the reference for the receiver amplifierat all frequencies other than the frequency established by the LCcircuit. At this selected frequency, which is the same as the frequencyof the decaying signal from the energy absorbing and radiating device,the emitter is held at a particular potential and therefore any signalof that selected frequency appearing at the base or control electrode isamplified.

Thus, the receiver component also is characterized by having no residualstored energy after a time, following cut-off of the energy field,substantially less than the decay time of the energy absorbing andradiating device 14.

The output signal from the receiver component may be utilized in anymanner desired. In the embodiment illustrated, this output signal isapplied to a Schmidt trigger circuit 264, silicone controlled rectifier,or other suitable device, the output of which functions to activate analarm 266.

It will be apparent that the transistors previously described may bereplaced by vacuum tubes, if desired. For example, the plateof a vacuumtube may be connected through the transformer winding 228' to a sourceof direct current potential, the control grid may be connected to thetransformer winding 230 through the parallel arrangement of a capacitorand resistor, and the cathode may be connected to ground through theseries tuned reference device 238.

With reference to the synchronized wave forms illustrated in FIG. 14, itwill be understood that while the transmitter is turned on, by virtue ofthe positive timing signals 268, the receiver component is turned off byvirtue of the negative timing signals 270. Similarly, after thetransmitter has been turned off, during the interval between adjacentsignals 268, the receiver component is turned on by the positive signals272 for receiving the decaying signal radiating from the energyabsorbing and radiating device.

Although the passivetuned resonant LC circuit 14 described hereinbeforeis the preferred form of energy absorbing and radiating device, becauseof its simplicity, economy and'capability of being provided in the formof a substantially two dimensional tag, it will be understood that otherforms of energy absorbing and radiating devices may be employed forapplications in which the foregoing advantages are not required.

In the. receiver component illustrated in FIG. 14, there is shown acapacitor 274 across the antenna transformer winding 248. This capacitormay be a physical element, or it may represent stray capacitance. ln anyevent, it is desirable to shunt the winding 'with a resistor 276 inorder to spoil the Q of the tuned circuit, in order for the receiver tohave no residual stored energy, as previously explained.

FIG. 15 illustrates a modified form of receiver component for use in theapparatus of FIG. 14. As in the previous embodiment, the receiverincludes an untuned high impedance amplifier 278 connected to thereceivingantenna 252 and having its reference electrode connectedto'ground through a low impedance resonant reference device 256.However, the output of the amplifier is fed to a coincidence gate 280,forming a part of the receiver, to which also is fed the timing signalsfrom the timer 232. These timing signals function to operate the gatesynthronously with the transmitter so as to select between thetransmitter signal from the transmitter antenna 226 and the decayingsignals from the energy absorbing and radiating device 14. Thus, duringthe time that the transmitter is on the timing signals to thecoincidence gate disables the latter so that the transmitter signalreceived by the receiving antenna 252 and amplified by the amplifier,are not passed through to the Schmidt trigger 264 to cause activation ofthe alarm 266. However, during the time the transmitter is shut off, thetiming signals function to enable the gate 280 to pass through to theSchmidt trigger the amplified decaying signal received by the receivingantenna 252 from the energy absorbing and radiating device 14.

As an alternative, through less efficient and therefore less desirableprocedure by which to provide a conventional receiver with thecharacteristic of having no residual stored energy, means such as adiode or other electrically actuated switch may be arranged to shunt oropen a tuned resonant antenna circuit, as by timing signals, during theperiod of time that the transmitter is turned on.

FIG. 11 illustrates another form of tuned resonant circuit usable withthe various forms of apparatus described hereinbefore. In thisembodiment two tuned resonant circuits are arranged to be inductivelycoupled to each other and preferably concealed in a tag 200 as in themanner previously described. Each tunedcircuit may be constructed in themanner of the tuned circuit illustrated in FIGS. 3 and 4. One of thetuned circuits comprises an open loop of electrically conductive metalforming cell 202 and capacitance 204. When employed with the apparatusof FIG. 8, it is tuned to the frequency of the transmitter 144. Theother tuned circuit comprises an open loop forming coil 206 andcapacitance 208 and it is tuned to the frequency of the receiver 154'which, for this purpose, is different from the frequency of thetransmitter. Although these frequeneies may be harmonics, it ispreferred that they not be, so as to insure against possible erroneousactivation of the receiver.

It is well known that when the tuned circuit 202, 204 is activated tooscillation, the closely coupled circuit 206, 208 will oscillate at thesame frequency as the tuned circuit 202, 204. Then, if the fluxenergizing the circuit 202, 204 is cut off, both resonant circuits 202,204 and 206, 208 will oscillate with a decaying wave at their ownnatural frequencies.

Accordingly, let it be assumed for purpose of explanation that thetransmitter 144 and tuned circuit 202, 204 are tuned to the frequency ofmegacycles and that the receiver and tuned circuit 206, 208 are tuned toa frequency of megacycles. Thus, when the tag 200 isplaced in theradiation field of the transmitter antenna and the tuned circuit 202,204 is excited during the time that the transmitter 144 is energized,the tuned circuit 206, 208 also will oscillate at the frequency of 100megacycles. Then, when the transmitter 144 is cut off, by operation ofthe clipper 146, the tuned circuit 202, 204 will transmit a damped waveat 100 megacycles and the tuned circuit 206,208 will transmit a dampedwave at its natural frequency of 120 megacycles. Since the receiver 154is tuned to the frequency of 120 megacycles, it will receive only thedamped wave from the tuned circuit 206,208.

Since the receiver 154 receives only the damped wave transmitted by thetuned circuit 206, 208 and is not activated by the transmitter 144 ortuned circuit 202, 204, it will be apparent that the receiver may beoperated continuously. Accordingly, the clipper circuit 156 in FIG. 8may be omitted.

When the tuned resonant circuit of FIG. 11 is employed with the type ofapparatus illustrated in FIGS. 1 and 6, it is necessary merely that thetime of complete collapse of the electromagnetic or electrostatic fieldbe adjusted, as by a potentiometer 42 in FIG. 1, to onehalf cycle of theresonant frequency of the tuned circuit 202, 204. Thus, during the timeinterval that the energy field is being radiated, both tuned circuits202, 204, and 206, 208 resonate at the frequency of the circuit 202,204. When the energy field is cut off, both tuned circuits resonate attheir own frequencies and therefore the damped wave from tuned circuit206, 208 is radiated for reception by the receiver, which may beoperated continuously.

In this latter regard, it will be understood that since the tunedresonant circuit 14 is activated by the apparatus of FIGS. 1 and 6 byadjusting the field collapse time to substantially one-half cycle of theresonant frequency of the tuned circuit 14, it is desirable that theassociated receiver 58 or 122 be activated only during the time thefield is cut off, in order to insure against possible false activationof the receiver during collapse of the field.

FIGS. 12 and 13 illustrate still further forms of tuned resonantcircuits which may be concealed in tags, and which are usable with thevarious forms of apparatus described hereinbefore. In FIG. 12 a printedcircuit or other suitable form of electrically conductive metal isarranged to form a pair of interconnected inductive loops 210 and 212. Alength of electrically conductive fusible material 214 bridges the metalbetween the loops 210 and 212, whereby to close the loop 210 and tocomplete the loop 212 which terminates at its spaced-apart ends in acapacitor 216. Loop 212 and capacitor 216 thus form an L-C circuit whichis arranged to resonate at the same frequency as receiver 58, 122 or154.

Destruction of the tuned resonant circuit is accomplished by placing itin a strong magnetic field of, for example, conventional 60 cyclehousehold alternating current. This causes a strong current to flow inthe closed loop 210, resulting in melting of the fusible link 214. Inthis regard, since the impedance of the closed loop 210 at 60 cycles issubstantially its direct current resistance, the current circulating inthe loop is much higher, for a given wattage, than it would be at higherfrequencies.

Upon melting of the link 214 the inductance of the open loop 212 isadded to loop 210. The combination of this increased inductance with thecapacitance 216 provides a tuned circuit which resonates at a lowerfrequency than previously provided by coil 212 and capacitor 216. Thus,although the-resonant circuit is destroyed effectively for use with thereceivers previously mentioned, it may be rc-used at the lower resonantfrequency in association with the similarly tuned receiver.

If it is desired to have the destroyed circuit re-usable at a frequencyhigher than that of coil 212 and capacitor 216, the arrangementillustrated in FIG. 13 may be provided. In this embodiment the fusiblelink 214 is made a part of the capacitor 216. Thus, upon melting of thelink the capacitance of the capacitor 216 is lowered, whereby theresonant frequency of coil 212 and capacitor 216 is increased.

The various forms of apparatus described hereinbefore also may beemployed to control the distribution of articles on a moving conveyorselectively to branch conveyors. For example, all articles intended fordistribution to one location may be identified by attachment of tagshaving tuned circuits of the same resonant frequency, while otherarticles intended for distribution to another location may be identifiedwith tags having tuned circuits of a different resonant frequency.Separate receivers, each corresponding in frequency to a different oneof the tuned circuits, thus may function to actuate appropriateequipment to divert the articles on the main conveyor to the appropriatebranch conveyors. Such an arrangement has many practical forms of use incommerce and industry. One such use is the selective distribution ofluggage at air terminals.

It will be apparent to those skilled in the art that various changes maybe made in the size, shape, arrangement, number, types and values ofparts described hereinbefore. For example, the number of stages ofclipper circuits may be varied to provide the waveform characteristicsdesired. The clipper circuits providing the substantially squarewaveforms illustrated may be replaced with sawtooth generators toprovide sawtooth waveforms, or with various other well known circuitsproviding other desired waveforms. The vacuum tube circuitry illustratedmay be replaced by transistor circuitry in well known manner.

Activation of the receiver for reception of the damped signal from thetuned resonant circuit has been exemplified hereinbefore by connectionof the output of clipper 56 to provide bias voltage for the first r.f.stage of the receiver. Another procedure is to connect the clipperoutput to a shorting tube, in the manner of shorting tube 40, which isconnected across the input resonant circuit of the receiver. In thiscase the secondary winding 54 is grounded at its upper end, in phaseopposition to winding 52, and the receiver is operated continuously butis not activated for reception of the tuned circuit signal except aftercollapse of the electrostatic (FIG. 1), or electromagnetic (FIG. 6)field, or after cut off of the transmitter (FIG. 8).

The foregoing and other changes may be made without departing from thespirit of this invention.

Having now described our invention and the manner in which it may beused, we claim:

1. Apparatus for activating an electric circuit, comprising a. resonantelectrical energy absorbing and radiating means tuned to a predeterminedfrequency and operable upon cut-off of an electric energy field in whichit is located to radiate a decaying electric signal at its tunedfrequency,

b. a radio frequency transmitter and a source of intermittent electricpotential connected to the transmitter for intermittently activating thelatter for producing an intermittent electric energy field in apredetermined space through which the energy absorbing and radiatingmeans may pass, the transmitter being characterized by having noresidual stored energy after a time, following cut-off of the energyfield, less than the decay time of the decaying signal,

c. electric signal detector means responsive only to said decayingelectric signal to produce an electric output signal, the detector meansbeing characterized by having no residual stored energy after a time,following cut-off of the energy field, less than the decay time of saiddecaying signal, and

d. means for connecting said output signalto an electric circuit to beactivated.

2. The apparatus of claim 1 wherein the resonant electrical energyabsorbing and radiating means comprises a passive LC tuned resonantcircuit.

3. The apparatus of claim 2 wherein the tuned resonant circuit isconfined between sheets of dielectric material forming a flat laminatedtag.

4. The apparatus of claim 2,wherein the tuned resonant circuit comprisestwo inductively coupled tuned resonant circuits, one tuned to thefrequency of the detector means and the other tuned to a differentfrequency, and wherein'the transmitter produces a radio frequency signaltuned -to the same frequency as said other tuned circuit.

5. The apparatus of claim 1 for activating an electric circuit fordetecting stolen merchandise, wherein the resonant electrical energyabsorbing and radiating means is attached to the merchandise.

6. The apparatus of claim 1 wherein the radio frequency transmitterincludes an oscillator having a reference electrode controlled by lowimpedance resonant means tuned to. a predetermined frequency, and energyabsorbing means is associated with the resonant means forreducing itseffective Q.

7. The apparatus of claim 6 wherein the resonant means comprises aseries tuned LC circuit, and the energy absorbing means comprisesresistance means arranged in shunt with the inductance of said LCcircuit.

8. The apparatus of claiml including electric timing 7 signal meansinterconnecting the radio frequency transmitter and the detector meansand operable to render the detector means incapable of producing saidoutput electric signal when the transmitter is turned on. i 9. Theapparatus of claim 1 wherein the radio frequency transmitter is tuned tothe samefrequency as the energy absorbing and radiating means, and theapparatus includes timer means synchronizing the transmitter anddetector means for activating the detector means only during the timesaid transmitter is cut off.

sity field, the LC circuit being tuned to a predetermined frequency andoperable upon cut-off of an "electric energy field in which it islocated to radiate a decaying electric signal at its tuned frequency,

b. electric field generating means for producing an intermittentelectric energy field in a predetermined space through which the LCtuned resonant circuit may pass, the generating means beingcharacterized by having no residual stored energy after a time,following cut-off of the energy field, less than the decay time of saiddecaying signal,

0. electric signal detector means responsive only to said decayingelectric signal to produce an electric output signal, the detector meansbeing characterized by having a residual stored energy after a time,

: following cut-off of the energy field, less than the decay time ofsaid decaying signal, and

d. means for connecting said output signal to an electric circuit to beactivated.

'14. The apparatus of claim 13 wherein the lower melting materialdivides the loop into a closed loop 10. The apparatus of claim 1 whereinthe electric signal detector means includes an amplifier having areference electrode controlled by low impedance resonant means tuned tothe frequency of the decaying signal of the resonant electrical energyabsorbing and radiating means, and energy absorbing means is associatedwith the resonant'means for reducing its effective Q.

11. The apparatus of claim 10 wherein the resonant means comprises aseries tuned LC circuit, and the energy absorbing means comprisesresistance means arranged in shunt with the inductance of said LCcircuit.

12. The apparatus of claim -1 wherein the radio frequency transmitterhasa cut-off time equal to about one-half cycle of the resonantfrequency of the energy absorbing and radiating means.

13. Apparatus for activating an electric circuit, comprising: v

a. a loop of electrically conductive material forming a passive LCcircuit, a portion of the loop comprising a material of lower meltingpoint capable of being melted under the influence of a currentdenportion and an open loop portion which defines the LC circuit.

15. The apparatus of claim 13 including means producing a currentdensity field capable of melting said material of lower melting point.

16. Apparatus for activating an electric circuit, comprising:

- a. resonant electrical energy absorbing and radiating means tuned to apredetermined frequency and operable upon cut-off of an electric energyfield in which it is located to radiate a decaying electric signal atits tuned frequency,

b. electric field generating means comprising a pair of spacedelectrostatic plates and a source of inter: mittent electric potentialconnected to the plates for intermittently charging the plates inopposite polarities, for producing an intermittent electric energy fieldin a predetermined space through which the energy absorbing andradiating means may pass, the generating means being characterized byhaving no residual stored energy after a time, following cut-off of theenergy field, less than the decay time of said decaying signal,

c. electric signal detector means responsive only to said decayingelectric signal to produce an electric output signal, the detector meansbeing characterized by having no residual stored energy after a time,following cut-off of the energy field, less than the decay time of saiddecaying signal, and

vd. means for connecting said output signal to an electric circuit to beactivated.

17. The apparatus of claim 16 including electric shorting switch meansconnected across the plates and operable to interconnect the plates whenthe latter are not being charged, whereby to accelerate discharge of theplates and collapsing of the electrostatic field.

18. Apparatus of activating an electric circuit, comprising:

a. resonant electrical energy absorbing a radiating means tuned to apredetermined frequency and operable upon cut-off of an electric energyfield in which it is located to radiate a decaying electric sig nal atits tuned frequency,

b. electric field generating means for producing an intermittentelectrostatic field in a predetermined space through which the energyabsorbing and radiating means may pass, the electrostatic field having acollapse time of about one-half cycle of the resonant frequency of theenergy absorbing and radiating means, the generating means beingcharace. means for connecting said output signal to an electric circuitto be activated.

energy field, less than the decay time of said decaying signal,

c. electric signal detector means responsive only to said decayingelectric signal to produce an electric terized by having no residualstored energy after a 5 output signal, the detector means beingcharactertime, following cut-off of the energy field, less than ized byhaving no residual stored energy after a the decay time of said decayingsignal, time, following cut-off of the energy field, less than electricsignal detector means responsive only to the decay time of said decayingsignal, and

said decaying electric signal to produce an electric (1. means forconnecting output signal to an electric output signal, the detectormeans being charactercircuit to be activated.

ized by having no residual stored energy after a 20. The method ofactuating an electric circuit for time, following cut-off of the energyfield, less than detecting stolen merchandise, comprising:

the decay time of said decaying signal, a. producing an intermittentelectric energy field in d. means for activating the detector means onlydura predetermined space through which customers ing the time said fieldis cut off, and are required to pass,

b. attaching to said merchandise for introduction into said space aresonant electrical energy absorbing 19. Apparatus for activating anelectric circuit, comprising:

a. resonant electrical energy absorbing and radiating and radiatingdevice tuned to a predetermined frequency and operable upon cut-off ofsaid field to radiate a decaying electric signal at its tuned fremeanstuned to a predetermined frequency and operable upon cut-off of anelectric energy field in which it is located to radiate a decayingelectric signal at its tuned frequency,

quency,

c. cutting off said energy field in a time less than the decay time ofsaid decaying electric signal,

d. detecting only said decaying electric signal to prob. electric fieldgenerating means comprising a pair duce an output electric signal,

of spaced electromagnetic coils and a source of ine. utilizing saidoutput electric signal to actuate an termittent electric potentialconnected to the coils electric circuit, and for intermittentlyenergizing said coils, for producf. subjecting the energy absorbing andradiating deing an intermittent electric energy field in a predevice toa radiant energy field sufficient to destroy termined space throughwhich the energy absorbits resonating characteristics prior to passageof the ing and radiating means may pass, the generating merchandisethrough said space, when said mermeans being characterized by having noresidual chandise has been purchased by a customer. stored energy aftera time, following cut off of the

1. Apparatus for activating an electric circuit, comprising a. resonantelectrical energy absorbing and radiating means tuned to a predeterminedfrequency and operable upon cut-off of an electric energy field in whichit is located to radiate a decaying electric signal at its tunedfrequency, b. a radio frequency transmitter and a source of intermittentelectric potential connected to the transmitter for intermittentlyactivating the latter for producing an intermittent electric energyfield in a predetermined space through which the energy absorbing andradiating means may pass, the transmitter being characterized by havingno residual stored energy after a time, following cut-off of the energyfield, less than the decay time of the decaying signal, c. electricsignal detector means responsive only to said decaying electric signalto produce an electric output signal, the detector means beingcharacterized by having no residual stored energy after a time,following cut-off of the energy field, less than the decay time of saiddecaying signal, and d. means for connecting said output signal to anelectric circuit to be activated.
 2. The apparatus of claim 1 whereinthe resonant electrical energy absorbing and radiating means comprises apassive LC tuned resonant circuit.
 3. The apparatus of claim 2 whereinthe tuned resonant circuit is confined between sheets of dielectricmaterial forming a flat laminated tag.
 4. The apparatus of claim 2wherein the tuned resonant circuit comprises two inductively coupledtuned resonant circuits, one tuned to the frequency of the detectormeans and the other tuned to a different frequency, and wherein thetransmitter produces a radio frequency signal tuned to the samefrequency as said other tuned circuit.
 5. The apparatus of claim 1 forActivating an electric circuit for detecting stolen merchandise, whereinthe resonant electrical energy absorbing and radiating means is attachedto the merchandise.
 6. The apparatus of claim 1 wherein the radiofrequency transmitter includes an oscillator having a referenceelectrode controlled by low impedance resonant means tuned to apredetermined frequency, and energy absorbing means is associated withthe resonant means for reducing its effective Q.
 7. The apparatus ofclaim 6 wherein the resonant means comprises a series tuned LC circuit,and the energy absorbing means comprises resistance means arranged inshunt with the inductance of said LC circuit.
 8. The apparatus of claim1 including electric timing signal means interconnecting the radiofrequency transmitter and the detector means and operable to render thedetector means incapable of producing said output electric signal whenthe transmitter is turned on.
 9. The apparatus of claim 1 wherein theradio frequency transmitter is tuned to the same frequency as the energyabsorbing and radiating means, and the apparatus includes timer meanssynchronizing the transmitter and detector means for activating thedetector means only during the time said transmitter is cut off.
 10. Theapparatus of claim 1 wherein the electric signal detector means includesan amplifier having a reference electrode controlled by low impedanceresonant means tuned to the frequency of the decaying signal of theresonant electrical energy absorbing and radiating means, and energyabsorbing means is associated with the resonant means for reducing itseffective Q.
 11. The apparatus of claim 10 wherein the resonant meanscomprises a series tuned LC circuit, and the energy absorbing meanscomprises resistance means arranged in shunt with the inductance of saidLC circuit.
 12. The apparatus of claim 1 wherein the radio frequencytransmitter has a cut-off time equal to about one-half cycle of theresonant frequency of the energy absorbing and radiating means. 13.Apparatus for activating an electric circuit, comprising: a. a loop ofelectrically conductive material forming a passive LC circuit, a portionof the loop comprising a material of lower melting point capable ofbeing melted under the influence of a current density field, the LCcircuit being tuned to a predetermined frequency and operable uponcut-off of an electric energy field in which it is located to radiate adecaying electric signal at its tuned frequency, b. electric fieldgenerating means for producing an intermittent electric energy field ina predetermined space through which the LC tuned resonant circuit maypass, the generating means being characterized by having no residualstored energy after a time, following cut-off of the energy field, lessthan the decay time of said decaying signal, c. electric signal detectormeans responsive only to said decaying electric signal to produce anelectric output signal, the detector means being characterized by havinga residual stored energy after a time, following cut-off of the energyfield, less than the decay time of said decaying signal, and d. meansfor connecting said output signal to an electric circuit to beactivated.
 14. The apparatus of claim 13 wherein the lower meltingmaterial divides the loop into a closed loop portion and an open loopportion which defines the LC circuit.
 15. The apparatus of claim 13including means producing a current density field capable of meltingsaid material of lower melting point.
 16. Apparatus for activating anelectric circuit, comprising: a. resonant electrical energy absorbingand radiating means tuned to a predetermined frequency and operable uponcut-off of an electric energy field in which it is located to radiate adecaying electric signal at its tuned frequency, b. electric fieldgenerating means comprising a pair of spaced electrostatic plates and asource of intermittent electric potential connected to the Plates forintermittently charging the plates in opposite polarities, for producingan intermittent electric energy field in a predetermined space throughwhich the energy absorbing and radiating means may pass, the generatingmeans being characterized by having no residual stored energy after atime, following cut-off of the energy field, less than the decay time ofsaid decaying signal, c. electric signal detector means responsive onlyto said decaying electric signal to produce an electric output signal,the detector means being characterized by having no residual storedenergy after a time, following cut-off of the energy field, less thanthe decay time of said decaying signal, and d. means for connecting saidoutput signal to an electric circuit to be activated.
 17. The apparatusof claim 16 including electric shorting switch means connected acrossthe plates and operable to interconnect the plates when the latter arenot being charged, whereby to accelerate discharge of the plates andcollapsing of the electrostatic field.
 18. Apparatus of activating anelectric circuit, comprising: a. resonant electrical energy absorbing aradiating means tuned to a predetermined frequency and operable uponcut-off of an electric energy field in which it is located to radiate adecaying electric signal at its tuned frequency, b. electric fieldgenerating means for producing an intermittent electrostatic field in apredetermined space through which the energy absorbing and radiatingmeans may pass, the electrostatic field having a collapse time of aboutone-half cycle of the resonant frequency of the energy absorbing andradiating means, the generating means being characterized by having noresidual stored energy after a time, following cut-off of the energyfield, less than the decay time of said decaying signal, c. electricsignal detector means responsive only to said decaying electric signalto produce an electric output signal, the detector means beingcharacterized by having no residual stored energy after a time,following cut-off of the energy field, less than the decay time of saiddecaying signal, d. means for activating the detector means only duringthe time said field is cut off, and e. means for connecting said outputsignal to an electric circuit to be activated.
 19. Apparatus foractivating an electric circuit, comprising: a. resonant electricalenergy absorbing and radiating means tuned to a predetermined frequencyand operable upon cut-off of an electric energy field in which it islocated to radiate a decaying electric signal at its tuned frequency, b.electric field generating means comprising a pair of spacedelectromagnetic coils and a source of intermittent electric potentialconnected to the coils for intermittently energizing said coils, forproducing an intermittent electric energy field in a predetermined spacethrough which the energy absorbing and radiating means may pass, thegenerating means being characterized by having no residual stored energyafter a time, following cut off of the energy field, less than the decaytime of said decaying signal, c. electric signal detector meansresponsive only to said decaying electric signal to produce an electricoutput signal, the detector means being characterized by having noresidual stored energy after a time, following cut-off of the energyfield, less than the decay time of said decaying signal, and d. meansfor connecting output signal to an electric circuit to be activated. 20.The method of actuating an electric circuit for detecting stolenmerchandise, comprising: a. producing an intermittent electric energyfield in a predetermined space through which customers are required topass, b. attaching to said merchandise for introduction into said spacea resonant electrical energy absorbing and radiating device tuned to apredetermined frequency and operable upon cut-off of said field toradiate a decaying electric signal at its tuned frequency, c. cuttingoff said energy field in a time less than the decay time of saiddecaying electric signal, d. detecting only said decaying electricsignal to produce an output electric signal, e. utilizing said outputelectric signal to actuate an electric circuit, and f. subjecting theenergy absorbing and radiating device to a radiant energy fieldsufficient to destroy its resonating characteristics prior to passage ofthe merchandise through said space, when said merchandise has beenpurchased by a customer.